This invention relates to a process for the recovery of zinc and other non-ferrous metal values from sulphidic material which also contains iron, and to produce a marketable iron product.
Bulk zinc, lead, copper and iron concentrates are produced at several locations throughout the world from the treatment of complex sulphide ores. In some locations, bulk concentrates are produced together with conventional concentrates of the individual metals. For other orebodies, the treatment of ore for metal recovery is only economical if a bulk concentrate, containing all the metals of interest, is produced. Such bulk concentrates are treated almost exclusively in Imperial Smelting Furnaces.
Although it has long been desired to develop a hydrometallurgical route for the treatment of bulk concentrates either to produce higher grade zinc or eliminate the sulphur dioxide emissions and sulphuric acid production requirements common to smelter operations, commercialization of such a route has not been successful.
Conventional zinc concentrates, containing about 50% Zn, 5 to 10% Fe, a maximum of 3.5% Pb and less than 1% Cu, can be treated by a dead roasting process to convert zinc sulphide to a mixture of zinc oxide and zinc ferrite. The zinc ferrite content of the calcine product is dependent on the iron content of the zinc concentrate and normally from 5 to 20% of the zinc is present as weak acid insoluble ferrite. Calcine is treated in a weak acid leach circuit to dissolve zinc oxide and to produce a solution from which zinc can be recovered by electrolysis after a purification step. Zinc ferrite however is unattacked in the weak acid leach and must be subjected to a separate hot acid leach to dissolve the ferrite. In this step, iron also dissolves and must be precipitated from solution before the dissolved zinc is recycled to the weak acid leach circuit, Several processes, such as jarosite precipitation, goethite precipitation, paragoethite precipitation and hematite precipitation have been developed for the precipitation of iron from hot acid leach solution.
Bulk concentrates have lower zinc content and higher lead, iron and copper contents than conventional zinc concentrates. Two problems exist in the treatment of bulk concentrate by a dead roasting process. Firstly, the low zinc and high iron content of the concentrate ensures that most or all of the zinc is converted to zinc ferrite which can only be treated by a hot acid leach. Insufficient zinc oxide is produced to neutralize the excess acid present in the hot acid leach solution. Secondly, the calcine produced when the combined copper and lead content of the concentrate is high tends to agglomerate in the roaster bed. However, small quantities of bulk concentrate have been successfully blended with conventional concentrates as a feed to a dead roast.
The New Brunswick Research and Productivity Council developed a sulphation roast process for the treatment of bulk concentrates, see J. Synnott et al., "Iron control in the RPC sulphation roast-leach process", in Iron Control in Hydrometallurgy, eds. J. E. Dutrizac and A. J. Monhemius, Ellis Horwood, Chichester, 1986, pp. 56-64. The concept of the sulphation roast was successfully demonstrated in a 10 t/d pilot plant, but the corrosive nature of the roaster off gas posed major equipment problems, and severe problems were experienced with the water and sulphate balance in the hydrometallurgical circuit used to treat the calcine.
Several attempts have been made to develop a hydrometallurgical chloride route for the treatment of bulk concentrates. The U.S. Bureau of Mines, see M. M. Wong et al., "Integrated operation of ferric chloride leaching, molten-salt electrolysis process for production of lead", U.S. Department of the Interior, Report of Investigation 8770, 1983, Dextec in Australia; see P. K. Everett, "The Dextec lead process", in Hydrometalllurgy Research, Development and Plant Practice, eds. K. Osseo-Asare and J. D. Miller, TMS, Warrendale, Pa., 1983, pp. 165-173", Elkem in Norway; see E. Andersen et al., "Production of base metals from complex sulphide concentrates by the ferric chloride route in a small continuous plant", in Complex Sulphide Ores, ed. M. J. Jones, IMM, London, 1980, pp. 186-192, BRGM in France; see C. Palvadeau, "Further developments in the electrolysis of lead from chloride electrolytes: pilot plant progress report", in Extraction Metallurgy '85, IMM, London, 1985, pp. 967-977, and CANMET in Canada; and see "The ferric chloride leach process for the treatment of bulk base metal sulphide concentrates", CANMET Report 89-4 (OP & J), CANMET, Energy Mines and Resources Canada, Ottawa, 1989, have each conducted major research programs. None of these processes has advanced to commercialization.
Sherritt Inc has been investigating the treatment of bulk zinc-lead-copper concentrates by pressure leaching since 1977. Several flowsheets have been developed. The flowsheet of FIG. 1 illustrates a single stage pressure leach in which the majority of the iron which was extracted from the concentrate was precipitated in the autoclave, primarily as plumbojarosite. Limestone and zinc dross were added to the leach solution to neutralize free acid present in the leach solution and precipitate residual soluble iron. The leach residue, containing lead, silver and iron, was digested in sulphuric acid to produce a lead/silver residue and a solution containing acid and iron. The leach solution was treated with limestone to produce an iron oxide/gypsum precipitate.
A major drawback of the single stage pressure leach process is the large amount of limestone required to neutralize acid and precipitate iron and the production of a large quantity of low grade iron oxide/gypsum residue which must be ponded.
Subsequent testwork led to the development of a two stage countercurrent pressure leach of bulk concentrate shown in FIG. 2, see M. E. Chalkley et al, "A Sherritt pressure leaching process: non-ferrous metals production from complex sulphide concentrates" presented at the Canada/EC Seminar on the Treatment of Complex Minerals; Ottawa, Oct. 12-14, 1982. In this case, the limestone requirements for the leach solution were reduced due to better acid utilization and more complete iron precipitation in the autoclave. Again, the majority of the dissolved iron was precipitated in the autoclave, primarily as plumbojarosite. The plumbojarosite residue was separated from the sulphidic fraction of the leach residue by flotation and subsequently treated with sulphur dioxide in a reduction leach to dissolve the plumbojarosite and produce a lead sulphate/silver residue. It was proposed to neutralize the reduction leach solution with lime or limestone and produce an iron oxide/gypsum residue.
The two stage countercurrent pressure leach offered some advantages over the single stage leach, but produced a similar poor quality iron residue.
The desire to produce a lead/silver residue directly in the pressure leach led to further development work and a two stage cocurrent pressure leach, typified in FIG. 3, see U.S. Pat. No. 4,505,744, issued Mar. 19, 1985. It is known that a high grade lead/silver residue can be produced from the high acid pressure leaching of bulk concentrate. Conditions in the autoclave must be chosen such that precipitation of dissolved iron is minimized. This can be achieved by ensuring that sufficient acid is present in solution at all times to minimize iron hydrolysis and precipitation. A high grade lead and silver residue can then be separated from the leach residue by flotation. While lead recovery from the bulk concentrate will be high, silver recovery will be dependent on the mineralogical form of silver in the bulk concentrate. Silver which is dissolved in the high acid leach may be precipitated from solution as silver sulphide which will report to the sulphidic fraction of the leach residue. The leach solution typically contains more than 50 g/L H.sub.2 SO.sub.4 and 10 to 15 g/L Fe and must be treated further to neutralize acid and precipitate iron before it can be forwarded to purification and electrolysis for zinc recovery. This treatment can be conveniently carried out in a second pressure leach step by reacting the solution with a conventional zinc concentrate under conditions which will favour the consumption of acid and the precipitation of iron. In order to minimize the loss of lead and silver, this zinc concentrate should preferably have a low lead and silver content. Iron is precipitated as a mixture of jarosites, other basic iron sulphates and hydrated iron oxides. The iron residue is separated from the leach residue by flotation and is ponded.
The two stage cocurrent leach process allows for the direct recovery of lead and silver from bulk concentrate in the pressure leach. However, two concentrates are required, with the ratio of bulk concentrate:zinc concentrate being about 0.67:1. Such a flowsheet may have merit for an orebody from which both conventional and bulk concentrates can be produced. As is the case with the previously described flowsheets, however, the iron residue is of low grade and must be ponded.
With increasing environmental concern about the disposal of iron residues, the two stage cocurrent leach flowsheet was expanded to include the precipitation of iron as hematite, FIG. 4, described in co-pending U.S. patent application No. 08/217,902. High acid leach solution was subjected to a neutralization/reduction stage with zinc concentrate, followed by neutralization with lime to produce a solution from which iron is precipitated in an autoclave as hematite. The hematite precipitation end solution is then treated with zinc concentrate in a low acid leach to neutralize acid and precipitate residual iron. The iron residue from the low acid leach is separated from the sulphidic fraction of the leach residue by flotation and is leached in spent electrolyte under atmospheric pressure to dissolve the precipitated iron compounds and produce a lead/silver residue which is combined with the lead/silver residue produced in the high acid leach. The leach solution from this iron dissolution step is recycled to the high acid leach, thus ensuring that substantially all of the iron leached from both concentrates is rejected as high grade hematite precipitate.
This flowsheet has a number of advantages. Because the iron residue produced in the low acid leach undergoes an iron dissolution step to recover lead and silver values, it is possible to increase the amount of bulk concentrate treated by replacing some or all of the zinc concentrate by bulk concentrate. The overall recovery of lead and silver will increase. A major advantage is the rejection of iron as an environmentally more acceptable and potentially marketable hematite product.
This flowsheet, however, has certain disadvantages. The iron in solution in the high acid leach discharge is mainly in the ferric state and the maximum concentration that can be maintained at an acceptable acid concentration will be less than 20 g/L. Consequently, the hematite precipitation circuit, which includes reduction and neutralization steps, must necessarily be large to treat the large volumes of solution produced. Since hematite precipitation is carried out at about 180.degree. C. and the reaction is endothermic, large quantities of steam are required for heating. Further, the flowsheet is relatively complex, including two separate feed preparation systems and two separate leach residue flotation steps.